1. Field of the Invention
The invention relates to a multi-layer, thermoplastic heat-sealable film suitable for conventional heat sealable packaging applications.
2. Brief Description of the Prior Art
Polypropylene film has become a useful and widely accepted packaging film because of its good moisture barrier, stiffness, high strength, and good optical properties. However, the optical properties of polypropylene films in certain applications are sometimes critical and improvement has long been needed.
U.S. Pat. No. 4,230,767 discloses a laminated packaging material with heat seal packaging properties composed of a polypropylene base layer and a surface layer of a two component blend of a propylene/ethylene copolymer with a variety of other olefins and copolymers including polybutylene copolymers. U.S. Pat. No. 4,230,767 does not disclose the use of a high melt index polybutylene in a laminated film with a polypropylene core.
U.S. Pat. Nos. 4,681,804; 4,677,025; 4,769,421; 4,343,852; and 4,766,178 disclose laminated films containing polypropylene and polybutylene, however none disclose the use of high melt index polybutylene as presently claimed.
U.S. Ser. No. 531,206, filed May 31, 1990 now U.S. Pat. No. 5,073,458 discloses a laminated film having a polypropylene core which is coated with layers of a blend of polypropylene random copolymer and a high melt index poly-1-butene. Applicant has found that a laminate having a polypropylene core and polypropylene random copolymer outer layers gives films having surprisingly better optical properties when a minor amount of a high melt index poly-1-butene is added either to the core or to the outer layers.